The present invention relates to a mirror for use in a microlithography projection exposure apparatus comprising a substrate and a reflective coating, a projection objective for use in a microlithography projection exposure apparatus, a microlithography projection exposure apparatus, and also a method for the correction of a surface form of a mirror comprising a substrate and a reflective coating.
Microlithography projection exposure apparatuses serve for producing microstructured components by means of a photolithographic method. In this case, a structure-bearing mask, the so-called reticle, is imaged onto a photosensitive layer with the aid of a projection optical unit. The minimum feature size that can be imaged with the aid of such a projection optical unit is determined by the wavelength of the imaging light used. The smaller the wavelength of the imaging light used, the smaller the structures that can be imaged with the aid of the projection optical unit. Nowadays, imaging light having the wavelength of 193 nm or imaging light having a wavelength in the extreme ultraviolet (EUV) range, i.e. 5 nm-30 nm is principally used. When imaging light having a wavelength of 193 nm is used, both refractive optical elements and reflective optical elements are employed within the microlithography projection exposure apparatus. By contrast, when imaging light having a wavelength in the range of 5 nm-30 nm is used, exclusively reflective optical elements (mirrors) are used.
In order to enable a good imaging of the structure-bearing mask onto the photosensitive layer, it is necessary for the imaging aberrations of the projection optical unit to be reduced as far as possible. Therefore, it is necessary to ensure the surface form of, in particular, the mirrors used within the projection optical unit with a high precision.
For this purpose, it is necessary to measure the optical properties of the individual mirror or of the projection optical unit highly precisely. This is done, for example, by means of an interferometric measurement method such as are described in EP 1306698 A1. Such measurements are often carried out under conditions which correspond as well as possible to the use conditions of the mirror. This concerns, in particular, the wavelength of the light used for measurement. The exact influence of a mirror on imaging light having a specific wavelength can be measured very exactly using light having this wavelength, in particular. In the case of measurement using light having a different wavelength, uncertainties can occur which result from the difference between the measurement wavelength and the imaging wavelength. A mirror for use with imaging light having a wavelength in the range of 5-30 nm is therefore often also measured using corresponding radiation. In order that the mirror reflects the measurement radiation, however, it is necessary to provide the mirror with a suitable reflective coating. This has the further advantage that the influences of the reflective coating on the surface form, such as introduction of stress, for example, are likewise taken into account during the measurement.
However, the measurement of the mirror with a reflective coating has the disadvantage that a correction of the surface form of the mirror becomes more difficult. Such a correction is carried out for example by suitable surface removal using ion beams. However, this can lead to an impairment of the reflectivity since the reflective coating has been altered by the removal.